[Unclenick]

Hodgdon says lot-to-lot variation in Varget burn rate is currently held to ±3%, so your new purchase may require you to tweak your loads to get to best performance equivalence. Simply tossing the old on top of the new could get you some bad loads because of as much as 6% difference in burn rate between the two lots is theoretically possible. If the old powder isn't very old and you blend the two very thoroughly you should not have a problem, but you will have to test loads with this new blended lot to look for matching performance.

What I usually do instead of blending is load up the last ten or fifteen rounds of the old powder, then do the same number with the new powder and head to the range. I fire over the chronograph alternating between the old and the new. That evens out the effects of fouling accumulation, barrel temperature and changing ambient conditions, if there are such. At home I separate the two sets of data manually and look for a difference in the average velocities of the two to learn if I need to adjust the charge of the new lot of powder to match the old. This has happened. Some of the early Varget, in particular, was slower than some of the current stuff. I don't think Hodgdon's QC was as good when Varget first came out as it is now.

The only other thing to watch out for in blending is age. If the old lot you have is old enough that it might begin to break down, putting it in with new powder can accelerate aging of the new powder. That's because acid breakdown products tend consume the stabilizer at the surface of the newer powder, accelerating the start of it's own breakdown. If I were mixing powder I would never mix twenty year old powder with new for fear of that happening. Ten year old powder wouldn't worry me, though.